class Animal:
    def __init__(self,species,sound):
     if species in['猫','狗','羊','猴子','老虎','大象']:
        self.species = species
        self.sound = sound
     else:
        self.species = None
        self.sound = None
    def speak(self):
        if self.species == None:
           print('动物园里没有动物,游戏结束!')
           return False
        else:
            print(self.species, self.sound)
            return True
